1. Field of the Invention
The present invention relates to an electrophoretic display (EPD) device and a method for fabricating the same, and more particularly to an EPD device with enhanced brightness and contrast ratio, reduced fabrication costs and short fabrication time, and a method for fabricating the same.
2. Discussion of the Related Art
An electrophoretic display (EPD) device is an image display device using a phenomenon that colloidal particles move to either one of the polarities when one pair of electrodes to which a voltage is applied are immersed into a colloidal solution. Compared to a liquid crystal display (LCD) device, such an EPD device has the advantages of wide viewing angle, high reflectivity, low power consumption, and the like, without using a backlight. It is thus expected to be widely used as a flexible display such as an electronic paper.
An EPD device has a structure in which an electrophoretic layer is interposed between two substrates. One of the two substrates is made of a transparent substrate and the other substrate is provided with an array of driving elements to display images in a reflective mode in which light entering from the outside of the device is reflected.
FIG. 1 is a view illustrating an EPD device 1 in accordance with the related art. As illustrated in FIG. 1, the EPD device 1 may include a first substrate 20 and a second substrate 40, thin-film transistors and a pixel electrode 18 formed on the first substrate 20, a common electrode 42 formed on the second substrate 40, an electrophoretic layer 60 formed between the first substrate 20 and the second substrate 40, and an adhesive layer 56 formed between the electrophoretic layer 60 and the pixel electrode 18.
The thin-film transistor may include a gate electrode 11 formed on the first substrate 20, a gate insulation layer 22 formed over the first substrate 20 having the gate electrode 11, a semiconductor layer 13 formed on the gate insulation substrate 22, and a source electrode 15 and a drain electrode 16 formed on the semiconductor layer 13. A passivation layer 24 is formed on the source electrode 15 and drain electrode 16 of the thin-film transistor.
The pixel electrode 18 is formed on the passivation layer 24 to apply a signal to the electrophoretic layer 60. A contact hole 28 is formed on the passivation layer 24, and the pixel electrode 18 is connected to the drain electrode 16 of the thin-film transistor through the contact hole 28.
A color filter layer 44 and a common electrode 42 are formed on the second substrate 40. The electrophoretic layer 60 is formed on the color filter layer 44 and the adhesive layer 56 is formed on the electrophoretic layer 60. The electrophoretic layer 60 may include a capsule 70 filled with white particles 74 and black particles 76 therein. When a signal is applied to the pixel electrode 18, then an electric field is generated between the common electrode 42 and the pixel electrode 18, and white particles 74 and black particles 76 inside the capsule 70 move in a direction of either the common electrode 42 or pixel electrode 18 by the electric field, thereby displaying an image.
For example, when a negative (−) voltage is applied to the pixel electrode 18 on the first substrate 20 and a positive (+) voltage is applied to the common electrode 42 on the second substrate 40, positively charged (+) white particles 74 are moved to the side of the first substrate 20 and negatively charged (−) black particles 76 are moved to the side of the second substrate 40. In this state, if light is incident from the outside, i.e., an upper portion of the second substrate 40, then the incident light is reflected from the black particles 76, thereby implementing a black color on the EPD device.
On the contrary, when a positive (+) voltage is applied to the pixel electrode 18 on the first substrate 20 and a negative (−) voltage is applied to the common electrode 42 on the second substrate 40, positively charged (+) white particles 74 are moved to the side of the second substrate 40 and negatively charged (−) black particles 76 are moved to the side of the first substrate 20. In this state, if light is incident from the outside, i.e., an upper portion of the second substrate 40, then the incident light is reflected from the white particles 74, thereby implementing a white color on the EPD device.
However, the EPD device 1 having the foregoing structure according to the related art may have the following problems.
First, the method of fabricating the EPD device according to the related art has difficulty in attaching the first substrate and the second substrate to each other.
In the EPD device according to the related art, the first substrate 20 and second substrate 40 are individually fabricated, and then the first substrate 20 and second substrate 40 are attached to each other by the adhesive layer 56 to complete the process. In other words, a thin-film transistor for driving a unit pixel and the pixel electrode 18 for applying an electric field to the electrophoretic layer are formed on the first substrate 20, and the common electrode 42, the color filter layer 44, the electrophoretic layer 60 and the adhesive layer 56 are formed on the second substrate 40. Then, the first substrate 20 and the second substrate 40 are attached to each other to complete the fabrication process.
However, the unit pixel of the EPD device is typically formed with a small size less than 150 micrometer in the height and width thereof, and thus it may be difficult to accurately align the electrophoretic layer with the unit pixel. If the electrophoretic layer is not accurately aligned with the first substrate formed with a thin-film transistor, then an electric field is not accurately delivered to electrophoretic particles, thereby causing a driving error.
Second, the method of fabricating the EPD device according to the related art has a complicated fabrication process.
The first substrate 20 and the second substrate 40 are separately fabricated through different processes and transferred by a transfer means to be attached to each other in an attachment process, and thus it may be difficult to utilize an in-line fabrication process.
Third, electrostatic discharge generated during the process of attaching the first substrate 20 and second substrate 40 may cause failure in the initial alignment of electrophoretic particles.
As discussed above, the common electrode 42, the color filter layer 44 and the electrophoretic layer 60 are formed on the second substrate 40, and the adhesive layer 56 is coated on the electrophoretic layer 60. A protection film is typically attached to the adhesive layer 56 in order to prevent the reduction of the adhesive force of the adhesive layer 56 and prohibit foreign materials from being adhered to the adhesive layer 56. However, the protection film should be peeled off from the second substrate 40 before being attached to the first substrate 20. In this case, electrostatic discharge generated during the process of peeling off the protection film may cause misalignment in the initial alignment of electrophoretic particles. The misalignment of electrophoretic particles may generate a moire defect with a comb-tooth-shaped pattern during the operation of the EPD device.
Fourth, the EPD device according to the related has a deteriorated brightness and contrast ratio.
In the EPD device according to the related art, the color filter layer 44 is typically formed on the second substrate 40 so as to implement colors. Colors are implemented as the light reflected from the white particles 74 passes through the color filter layer 44.
In case of using the color filter layer 44, light passes through the color filter layer 44 two times: once when being incident onto the white particles 74 from the outside, and the other when being emitted to the outside after being reflected from the white particles 74. Accordingly, the brightness of the EPD device is lowered due to an absorption of light by the color filter layer 44.
For instance, when light passes through the color filter layer 44, about 46% of the light is absorbed. As a result, about 7.7% of the light incident from the outside is emitted to the outside after it passes through the color filter layer 44 and the electrophoretic layer 60, which greatly reduces the brightness of the EPD device.